This invention generally relates to optical components such as an optical monitoring device. More particularly this invention relates to tunable optical bandpass filters.
It is essential to monitor optical signals for highly reliable WDM systems. However, it becomes difficult to resolve each individual optical channel for power measurement because the individual channel spacing in DWDM systems decreases in order to increase transmission capacity. A very narrow band optical filter may be used to extract one channel in the DWDM signals. Bulk optic narrow band optical filters typically require advanced technologies and are typically expensive.
Bulk optic narrow band optical filters exist that use a Fabry Perot filter having movable mirrors. The movable mirrors cause resonant frequency changes in the optical signal. These narrow band optical filters may create a transmission spectrum narrow enough to extract one channel of a DWDM filter for analysis. The construction of these precise movable mirrors is typically expensive and the filter may have high insertion losses.
All-fiber optical filters typically have lower insertion losses on the optical signal than bulk optic filters. Most all-fiber optical filters, however have a bandwidth of a wide band tunable filter that is usually several nanometers. The corresponding transmission response is an order of magnitude wider than that of the required bandwidth for performance monitoring of the DWDM signal.
All-fiber type filters typically employ a filtering effect using the relation between modes of light propagating in an optical fiber. In principle, a light wave propagates through the core of optical fiber as the light wave totally reflects at an interface between the core and cladding of the optical fiber. On the other hand, the light wave has difficulty in propagating through the cladding of the optical fiber because the jacket surrounding the cladding is highly absorptive and its refractive index is higher than that of the cladding, which causes strong attenuation.
If the jacket is strippedxe2x80x94that is, if the cladding is exposed in the airxe2x80x94the light wave can propagate farther because the light wave is totally reflected at the interface between the cladding and air due to the higher refractive index of the cladding than that of air.
The mode for a light wave that propagates satisfying the total reflection condition at the interface of core and cladding may be referred to as a xe2x80x9ccore mode.xe2x80x9d The mode for a light wave that propagates satisfying the total reflection condition at the boundary surface of cladding and surrounding air, while failing to satisfy the total reflection condition at core/cladding interface, is called xe2x80x9ccladding mode.xe2x80x9d
In core mode, most of the energy of the optical signal is distributed in the core. In cladding mode, most of the energy of the optical signal is distributed in the cladding.
Various methods, apparatuses, and systems are described for routing an optical signal through the optical filter multiple times.
Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.